Laser engineering of titanium dioxide for ambient-processed perovskite solar cells

Abstract

Perovskite solar cells (PSCs) are considered one of the most promising next-generation photovoltaic technologies. Thermal annealing is a critical process in the fabrication of metal oxides such as TiO2, acting as the electron transport layer (ETL) for efficient PSCs. However, conventional annealing methods to fabricate TiO2 is a time-consuming batch process, involving the uses of a hotplate, furnace, or oven, over a period of 1–3 hours with a time-consuming cooling period that is challenging to enable a rapid and in-line production process. In addition, these conventional annealing methods are commonly limited to below 550 °C to avoid bending and breakage of glass substrates. Several studies have suggested that an annealing temperature beyond 600 °C improves the crystallinity of the TiO2 films and interconnection between the TiO2 nanoparticles, which enhances the performance of the PSCs. Therefore, there is a need to develop an alternative annealing process, enabling the rapid and scalable production of high-quality metal oxide films for the commercialization of PSCs. This thesis work is aiming to demonstrate rapid laser-annealing processes for the fabrication of high-quality TiO2 films for both mesoscopic and planar PSCs. Firstly, a one-step pulsed fiber laser (1070 nm wavelength) process is developed to fabricate both mesoporous and compact TiO2 films on tin-doped indium oxide (ITO) glass for mesoscopic PSCs. It is found that 1 min of laser irradiation with a peak temperature up to 595 °C can successfully induce the crystallization of the compact TiO2 film, remove the organic binders in the TiO2 paste and generate the necking between the TiO2 nanoparticles simultaneously. The average power conversion efficiency (PCE) obtained for the mesoscopic PSCs by the laser irradiation for 1 min is equivalent to that by a furnace treatment for 2 hours. For this laser process, it takes 1 min to process 3 cm2 (1 cm2 in 20 s) of the compact and mesoporous TiO2 films coated on ITO-glass for mesoscopic PSCs. Secondly, a continuous-wave fiber laser (1070 nm wavelength) process with a unique design of power ramping and beam configuration is developed to achieve ultrafast and scalable processing of compact TiO2 films for planar PSCs. The laser process involves initial high-power irradiation to rapidly increase the annealing temperature followed by low-power irradiation to stabilize the peak temperature, with a laser spot size up to 19.6 cm2. This laser process significantly reduces the processing time from 3 hours (including the cooling-down period) by the furnace process at 500 °C to 18.5 s by the laser irradiation at a peak temperature up to 800-850 °C. Then, a capability of using this laser process to anneal two stacked layers of the substrates coated with TiO2 films is demonstrated and can potentially achieve an in-line production rate of over 43 cm2 min-1 (1 cm2 in ~1.4 s). This technique offers a new opportunity for scalable annealing and mass production of thin metal oxide films. In addition, planar PSCs with the TiO2 films annealed under optimal laser conditions show the improved PCE and reduced hysteresis behavior compared to that of the furnace-annealed samples. Systematic characterizations and thermal measurements are also carried out to understand the laser-annealing process and the cause of the improvement in the device performance by the laser process. In addition to the development of the laser processes, a study on improving the PCE and stability of ambient-processed PSCs is carried out since all the PSCs in this thesis work were fabricated in ambient air under a high relative humidity (RH) of 50-70%. The effect of using a green antisolvent ethyl acetate with various halide additives on the performance of ambient-processed planar PSCs is investigated with a one-step deposition process. Differences in microstructures, crystallinity, surface chemistry, light absorption and optoelectronic properties for the green antisolvent and ambient-processed perovskite films with the addition of PbCl2, PbBr2, and excess MAI are observed, in comparison to the standard stoichiometric perovskite film. Ambient-processed planar PSCs with PbCl2 addition remarkably improves the PCE of the pristine device by 17.3% and achieves a fill factor up to 76.3%. Addition of PbCl2 also enhances the long-term stability of the devices and retains 81% of their initial device PCEs after storage in ambient air under an RH of 50-60% for 28 days.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Zhu Liu (Supervisor) & Michele Curioni (Supervisor)